Positive effects of ants on host trees are critical in years of low reproduction and not influenced by liana presence

Tropical tree-climbing lianas form paths that benefit foraging of dominant ants which might protect the host tree against herbivores. In contrast, lianas are often associated with negative effects on growth and reproduction of host trees due to light obstruction, structural stress and other negative effects. It is unclear if dominant ants could mitigate the negative effects of lianas on host plants. We investigated how lianas and carton nest ants (Azteca chartifex) affected herbivory and reproductive structures of the host tree Byrsonima sericea. Considering 68 trees, almost half of them were naturally colonized by A. chartifex nests (32 trees). We removed lianas from half of the trees (34), establishing a factorial sampling design between A. chartifex and liana presence. We sampled ants and leaf herbivory before and after removing lianas, and measured plant fitness in two consecutive years after removing lianas. Liana removal had no effect on A. chartifex foraging, on leaf herbivory and flower-fruit conversion of host plants. However, A. chartifex decreased leaf herbivory and increased B. sericea flower-fruit conversion irrespective of lianas presence. A noticeable positive effect of ants was detected only in the second year of the experiment, consistently on all plants at each experimental level. The reproductive conditions of the first year resulted in most plants with more than 75% flower-to-fruit conversion success, regardless of the presence of A. chartifex, a success sustained only on those ant-colonized plants in the second year. Our results contribute to understanding multi-trophic interactions in tropical forest canopies as we demonstrated i) that dominant arboreal ants can benefit plants even in a non-obligatory interaction and ii) that the influence of lianas on its host tree is context-dependent, presenting even neutral effects depending on habitat type and species involved.     

Herbivory on Temperate Rainforest Seedlings in Sun and Shade: Resistance,Tolerance and Habitat Distribution

Differential herbivory and/or differential plant resistance or tolerance in sun and shade environments may influence plant distribution along the light gradient. Embothrium coccineum is one of the few light-demanding tree species in the temperate rainforest of southern South America, and seedlings are frequently attacked by insects and snails. Herbivory may contribute to the exclusion of E. coccineum from the shade if 1) herbivory pressure is greater in the shade, which in turn can result from shade plants being less resistant or from habitat preferences of herbivores, and/or 2) consequences of damage are more detrimental in the shade, i.e., shade plants are less tolerant. We tested this in a field study with naturally established seedlings in treefall gaps (sun) and forest understory (shade) in a temperate rainforest of southern Chile. Seedlings growing in the sun sustained nearly 40% more herbivore damage and displayed half of the specific leaf area than those growing in the shade. A palatability test showed that a generalist snail consumed ten times more leaf area when fed on shade leaves compared to sun leaves, i.e., plant resistance was greater in sun-grown seedlings. Herbivore abundance (total biomass) was two-fold greater in treefall gaps compared to the forest understory. Undamaged seedlings survived better and showed a slightly higher growth rate in the sun. Whereas simulated herbivory in the shade decreased seedling survival and growth by 34% and 19%, respectively, damaged and undamaged seedlings showed similar survival and growth in the sun. Leaf tissue lost to herbivores in the shade appears to be too expensive to replace under the limiting light conditions of forest understory. Following evaluations of herbivore abundance and plant resistance and tolerance in contrasting light environments, we have shown how herbivory on a light-demanding tree species may contribute to its exclusion from shade sites. Thus, in the shaded forest understory, where the seedlings of some tree species are close to their physiological tolerance limit, herbivory could play an important role in plant establishment.     

Variation in the composition and activity of ants on defense of host plant <Emphasis Type="Italic">Turnera subulata</Emphasis> (Turneraceae): strong response to simulated herbivore attacks and to herbivore’s baits

Plants with extrafloral nectaries attract a variety of ant species, in associations commonly considered mutualistic. However, the results of such interactions can be context dependent. Turnera subulata is a shrub widely distributed among disturbed areas which has extrafloral nectaries at the base of leaves. Here, we evaluated whether the ants associated with T. subulata (i) vary in space and/or time; (ii) respond to simulated herbivory, and (iii) reduce herbivory rates. For this, we quantified the abundance and species richness of ants associated with T. subulata throughout the day in six different sites and the defensive capability of these ants under simulated herbivory in the leaves and stems of T. subulata plants (N = 60). We also checked the proportion of the lost leaf area and quantified leaf damage by chewing herbivores in the host plant. We found that a total of 21 ant species associated with the host plant. Species composition showed significant variation across the sampled sites and throughout the day. Visitation rates and predation by ants were higher in plant stems than in leaves. In general, herbivory rates were not correlated with ant association or activity, with the exception of the proportion of leaf area consumed; there was a significant lower herbivory rate on plants in which ants defended the leaves. Our results suggest that the benefits of association may depend on the ecological context. This context dependence may mask the correlation between the defense of ants and herbivory rates.     

Cytogenetic and Molecular Analyses Reveal a Divergence between Acromyrmex striatus (Roger, 1863) and Other Congeneric Species: Taxonomic Implications

The leafcutter ants, which consist of Acromyrmex and Atta genera, are restricted to the New World and they are considered the main herbivores in the neotropics. Cytogenetic studies of leafcutter ants are available for five species of Atta and 14 species of Acromyrmex, both including subspecies. These two ant genera have a constant karyotype with a diploid number of 22 and 38 chromosomes, respectively. The most distinct Acromyrmex species from Brazil is A. striatus, which is restricted to the southern states of Santa Catarina and Rio Grande do Sul. Several cytogenetic and phylogenetic studies have been conducted with ants, but the karyotypic characterization and phylogenetic position of this species relative to leafcutter ants remains unknown. In this study, we report a diploid number of 22 chromosomes for A. striatus. The phylogenetic relationship between A. striatus and other leafcutter ants was estimated based on the four nuclear genes. A. striatus shared the same chromosome number as Atta species and the majority of metacentric chromosomes. Nuclear data generated a phylogenetic tree with a well-supported cluster, where A. striatus formed a different clade from other Acromyrmex spp. This combination of cytogenetic and molecular approaches provided interesting insights into the phylogenetic position of A. striatus among the leafcutter ants and the tribe Attini.     

Aluminum toxicity to tropical montane forest tree seedlings in southern Ecuador: response of biomass and plant morphology to elevated Al concentrations

Aims

In acid tropical forest soils (pH <5.5) increased mobility of aluminum might limit aboveground productivity. Therefore, we evaluated Al phytotoxicity of three native tree species of tropical montane forests in southern Ecuador.

Methods

An hydroponic dose-response experiment was conducted. Seedlings of Cedrela odorata L., Heliocarpus americanus L., and Tabebuia chrysantha (Jacq.) G. Nicholson were treated with 0, 300, 600, 1200, and 2400 μ M Al and an organic layer leachate. Dose-response curves were generated for root and shoot morphologic properties to determine effective concentrations (EC).

Results

Shoot biomass and healthy leaf area decreased by 44 % to 83 % at 2400 μ M Al, root biomass did not respond (C. odorata), declined by 51 % (H. americanus), or was stimulated at low Al concentrations of 300 μ M (T. chrysantha). EC10 (i.e. reduction by 10 %) values of Al for total biomass were 315 μ M (C. odorata), 219 μ M (H. americanus), and 368 μ M (T. chrysantha). Helicarpus americanus, a fast growing pioneer tree species, was most sensitive to Al toxicity. Negative effects were strongest if plants grew in organic layer leachate, indicating limitation of plant growth by nutrient scarcity rather than Al toxicity.

Conclusions

Al toxicity occurred at Al concentrations far above those in native organic layer leachate.     

Observations on the Ecology of Weaver Ants (Oecophylla smaragdina Fabricius) in a Thai Mangrove Ecosystem and Their Effect on Herbivory of Rhizophora mucronata Lam.

Ants of the genus Oecophylla are predators of other insects and are able to protect a variety of terrestrial plants against pest insects; however, observations on the ecology of these ants in mangrove forests are lacking. General observations on the ecology of Oecophylla smaragdina were carried out in a Thai mangrove forest to determine if these ants can protect their host plants in less favorable mangrove habitats. Leaf herbivory and the density of O. smaragdina ants were measured on Rhizophora mucronata trees at two sites. The results showed a negative correlation between ant density and herbivory. At both sites, the mean percent damaged leaf area was more than four times higher on trees without ants compared to “ant‐trees.” A significant negative correlation was found between tree mean percent leaf damage and the density of ants on the tree. Furthermore, on trees with ants, there was less herbivory on leaves close to ant nests compared to other leaves on the tree. Most damage was caused by chrysomelid beetles (62%) and sesarmid crabs (25%) and both types of herbivory were significantly reduced on ant‐trees.     

Do carbon-based defences reduce foliar damage? Habitat-related effects on tree seedling performance in a temperate rainforest of Chiloé Island, Chile

Carbon-based secondary compounds (CBSCs), such as phenols or tannins, have been considered as one of the most important and general chemical barriers of woody plants against a diverse array of herbivores. Herbivory has been described as a critical factor affecting the growth and survival of newly established tree seedlings or juveniles then, the presence of secondary metabolites as defences against herbivores should be a primary strategy to reduce foliar damage. We examined whether light-induced changes in leaf phenolic chemistry affected insect herbivory on seedlings of two rainforest tree species, Drimys winteri (Winteraceae) and Gevuina avellana (Proteaceae). Seedlings of both species were planted under closed canopy and in a canopy gap within a large remnant forest patch. Half of the seedlings in each habitat were disinfected with a wide-spectrum systemic insecticide and the other half were used as controls. Seedling growth, survival, and foliar damage (estimated by an herbivory index) due to insect herbivores were monitored over a period of 16 months (December 2001–April 2003). The total leaf content of phenols and condensed tannins were assessed in seedlings from both habitats. As expected, access to light induced a greater production of CBSCs in seedlings of both tree species, but these compounds did not seem to play a significant defensive role, as seedlings grown in gaps suffered greater leaf damage than those planted in forest interior. In addition, in both habitats, seedlings without insecticide treatment suffered a greater foliar damage than those with insecticide, especially 16 months after the beginning of the experiment. Canopy openness and herbivory had positive and negative effects, respectively, on seedling growth and survival in both tree species. In conclusion, despite the higher levels of defence in tree-fall gap, the higher densities of herbivore override this and lead to higher damage levels.     

Potential impact of the leaf‐cutting ant Acromyrmex lobicornis on conifer plantations in northern Patagonia,Argentina

• 1 The economic losses associated with crop damage by invasive pests can be minimized by recognizing their potential impact before they spread into new areas or crops.
• 2 We experimentally evaluated the preferences of the leaf‐cutting ant Acromyrmex lobicornis (Hymenoptera: Formicidae) for the most common conifer species commercially planted in northern Patagonia, Argentina. The areas of potential forest interest in this region and the geographical range of this ant overlap. We performed field preference tests and monitored the level of ant herbivory on planted conifer seedlings next to nests.
• 3 Acromyrmex lobicornis preferred some conifer species and avoided foraging on others. Pseudotsuga menziesii and Austrocedrus chilensis were the less preferred species, Pinus ponderosa and Pinus contorta were the most preferred by A. lobicornis.
• 4 The item mostly selected by ants was young needles from P. contorta. This species was also the pine mostly defoliated. Seedlings without ant‐exclusion showed a mean±SE of 60±5% defoliation during the sampling period. Pinus ponderosa was less defoliated; control seedlings showed a mean±SE of 8.5±1% of leaf damage in the sampling period.
• 5 The present study shows how the use of simple field tests of leaf‐cutting ant preferences could allow an improved selection of appropriate conifer species for future plantations in areas where leaf‐cutting ants are present.

     

Factors Affecting the Use of Reforested Sites by Reptiles in Cleared Rainforest Landscapes in Tropical and Subtropical Australia

Despite recent efforts to reforest cleared rainforest landscapes, in Australia and elsewhere, the value of reforested sites for rainforest‐dependent reptiles is unknown. We surveyed the occurrence of reptiles in a range of reforestation types (monoculture and mixed‐species timber plantations, diverse “ecological restoration” plantings and regrowth), as well as reference sites in pasture and rainforest, in tropical and subtropical Australia. We recorded 29 species of reptiles from 104 sites, including 15 rainforest‐dependent species. Most rainforest reptiles were strongly associated with complex microhabitats (tree trunks, logs, rocks). The richness and abundance of rainforest‐dependent reptiles varied between the different types of reforestation and between regions. In the tropics, rainforest reptiles were recorded in old timber plantations and ecological restoration plantings but not in young timber plantations or regrowth. Rainforest reptiles were recorded in few reforested sites in the subtropics. The occurrence of rainforest‐dependent reptiles in reforested sites appears to be influenced by (1) habitat structure; (2) proximity to source populations in rainforest; and (3) biogeography and historical differences in the extent of rainforest. Restoration of cleared land for rainforest‐dependent reptiles may require the development, or deliberate creation, of complex structural attributes and microhabitats in reforested sites. Where reforested sites are located away from rainforest, recolonization by rainforest reptiles may require the construction of corridors of suitable habitat between reforested sites and rainforest or the translocation of reptiles to reforested sites.     

Conservation of an Ant–plant Mutualism in Native Forests and Ecologically‐managed Tree Monocultures

The Brazilian Atlantic Forest has been replaced by homogeneous tree monocultures with potentially drastic effect on ecological interactions. We expect that ecologically‐managed tree monocultures, however, can help to mitigate this impact. Here, we carried out an experiment with Inga vera (Fabaceae), an extrafloral nectary bearing plant, to test if the efficiency of ants as anti‐herbivory defense is affected by the replacement of its natural habitat (Araucaria Forest) by ecologically‐managed tree monocultures (plantations of Araucaria, Pinus, and Eucalyptus). Seedlings of Inga vera were transplanted to three patches of each habitat and ants were excluded from half of the plants. The abundance of ants and herbivores was low, similar among habitats, and exhibited temporal asynchrony. Number of herbivores and accumulated herbivory levels were lower in plant with ants. Rates of herbivory were extremely low and lower for young leaves than for mature leaves. The presence of ants did not affect plant performance traits measured by their growth in height, and their final numbers of leaves and leaflets. Contrary to what might be expected, ant‐protected plants produced fewer leaves and leaflets than unprotected ones. In conclusion, Inga vera‐ant interaction was similar between its natural habitat and the tree monocultures, indicating that potentially both species diversity and ecological processes can be conserved in ecologically‐managed tree monocultures.     

Biodiversity recovery during rainforest reforestation as indicated by rapid assessment of epigaeic ants in tropical and subtropical Australia

There is growing interest in the potential for reforestation to assist the recovery of rainforest biodiversity. There is also a need to identify taxonomically tractable groups for use as cost‐effective indicators when monitoring the status of biodiversity within reforested sites. Insects are an important component of terrestrial biodiversity but often require considerable resources to sample at species level. Ant genera and generic‐based functional groups have been suggested as possible indicators of environmental disturbance. Here we ask to what extent the development of biodiversity is indicated by epigaeic ant genera and functional groups, across different types of reforestation in tropical and subtropical Australia. In each region, we used pitfall traps to sample the ants in replicate sites of: unmanaged regrowth, monoculture and mixed species plantations and ‘ecological restoration’ plantings, together with reference sites in pasture and rainforest. We recorded 35 epigaeic ant genera (and 4623 individuals) from 50 tropical sites, and 39 genera (and 9904 individuals) from 54 subtropical sites, with 47 genera overall. Community composition of both genera and functional groups differed between pasture and rainforest, although many genera were widespread in both. Reforested sites were intermediate between pasture and rainforest in both regions, and showed a gradient associated with decreasing grass and increasing tree and litter cover. Older monoculture plantations and ecological restoration plantings had the most rainforest‐like ant assemblages, and mixed‐species cabinet timber plots the least, of the reforested sites. We conclude that ground‐active ant genera and functional groups sampled in rapid surveys by pitfall‐trapping showed only a modest ability to discriminate among different types of reforestation. Species‐level identification, perhaps together with expanded sampling effort, could be more informative, but would require resourcing beyond the scope of rapid assessments.     

Plant defense, herbivory, and the growth of Cordia alliodora trees and their symbiotic Azteca ant colonies

The effects of herbivory on plant fitness are integrated over a plant??s lifetime, mediated by ontogenetic changes in plant defense, tolerance, and herbivore pressure. In symbiotic ant?Cplant mutualisms, plants provide nesting space and food for ants, and ants defend plants against herbivores. The benefit to the plant of sustaining the growth of symbiotic ant colonies depends on whether defense by the growing ant colony outpaces the plant??s growth in defendable area and associated herbivore pressure. These relationships were investigated in the symbiotic mutualism between Cordia alliodora trees and Azteca pittieri ants in a Mexican tropical dry forest. As ant colonies grew, worker production remained constant relative to ant-colony size. As trees grew, leaf production increased relative to tree size. Moreover, larger trees hosted lower densities of ants, suggesting that ant-colony growth did not keep pace with tree growth. On leaves with ants experimentally excluded, herbivory per unit leaf area increased exponentially with tree size, indicating that larger trees experienced higher herbivore pressure per leaf area than smaller trees. Even with ant defense, herbivory increased with tree size. Therefore, although larger trees had larger ant colonies, ant density was lower in larger trees, and the ant colonies did not provide sufficient defense to compensate for the higher herbivore pressure in larger trees. These results suggest that in this system the tree can decrease herbivory by promoting ant-colony growth, i.e., sustaining space and food investment in ants, as long as the tree continues to grow.     

Phylogenetically diverse AM fungi from Ecuador strongly improve seedling growth of native potential crop trees

In many deforested regions of the tropics, afforestation with native tree species could valorize a growing reservoir of degraded, previously overused and abandoned land. The inoculation of tropical tree seedlings with arbuscular mycorrhizal fungi (AM fungi) can improve tree growth and viability, but efficiency may depend on plant and AM fungal genotype. To study such effects, seven phylogenetically diverse AM fungi, native to Ecuador, from seven genera and a non-native AM fungus (Rhizophagus irregularis DAOM197198) were used to inoculate the tropical potential crop tree (PCT) species Handroanthus chrysanthus (synonym Tabebuia chrysantha), Cedrela montana, and Heliocarpus americanus. Twenty-four plant-fungus combinations were studied in five different fertilization and AMF inoculation treatments. Numerous plant growth parameters and mycorrhizal root colonization were assessed. The inoculation with any of the tested AM fungi improved seedling growth significantly and in most cases reduced plant mortality. Plants produced up to threefold higher biomass, when compared to the standard nursery practice. AM fungal inoculation alone or in combination with low fertilization both outperformed full fertilization in terms of plant growth promotion. Interestingly, root colonization levels for individual fungi strongly depended on the host tree species, but surprisingly the colonization strength did not correlate with plant growth promotion. The combination of AM fungal inoculation with a low dosage of slow release fertilizer improved PCT seedling performance strongest, but also AM fungal treatments without any fertilization were highly efficient. The AM fungi tested are promising candidates to improve management practices in tropical tree seedling production.     

Ant–aphid interactions on Asclepias syriaca are mediated by plant genotype and caterpillar damage

The means by which plant genotypes influence species interactions and arthropod community structure remain poorly understood. One potential, but largely unstudied mechanism is that occurring through plant genetic variation in induced responses to herbivory. Here we test whether induced responses to leaf damage and genotypic variation for induction in Asclepias syriaca influence interactions among Formica podzolica ants, the ant‐tended aphid Aphis asclepiadis, and the untended aphid Myzocallis asclepiadis. In so doing, we assess genetic variation in plant‐mediated interactions among different herbivore guilds. We conducted a three‐way factorial field experiment manipulating plant genotype, leaf damage by specialist monarch caterpillars Danaus plexippus, and ant presence, and documented effects on aphid and ant abundances. Leaf damage increased Aphis abundance in both the presence and absence of ants and Myzocallis abundance under ant exclusion. In the presence of ants, leaf damage decreased Myzocallis abundance, likely due to effects on ant–Myzocallis interactions; ants showed a positive association with Myzocallis, leaf damage increased the strength of this association (425% more ants per aphid), and this in turn fed back to suppress Myzocallis abundance. Yet, these aggregate effects of leaf damage on Myzocallis and ants were underlain by substantial variation among milkweed geno types, with leaf damage inducing lower aphid and ant abundances on some genotypes, but higher abundances on others. As a consequence, a substantial fraction of the variation in leaf damage effects on ants (R² =0.42) was explained by milkweed genetic variation in the strength and sign of leaf damage effects on Myzocallis. Although plant genetic variation influenced Aphis abundance, this did not translate into genetic variation in ant abundance, and leaf damage did not influence Aphis–ant interactions. Overall, we show that variation in induced responses to herbivory is a relevant condition by which plant genotype influences interactions in plant‐centered arthropod communities and provide novel results of effects on the third trophic level.     

Leaf synchrony and insect herbivory among tropical tree habitat specialists

Growth defense tradeoff theory predicts that plants in low-resource habitats invest more energy in defense mechanisms against natural enemies than growth, whereas plants in high-resource habitats can afford higher leaf loss rates. A less-studied defense against herbivores involves the synchrony of leaf production, which can be an effective defense strategy if leaf biomass production exceeds the capacity of consumption by insects. The aim of this study was to determine whether leaf synchrony varied across habitats with different available resources and whether insects were able to track young leaf production among tree habitat specialists in a tropical forest of French Guiana. We predicted that high-resource habitats would exhibit more synchrony in leaf production due to the low cost and investment to replace leaf tissue. We also expected closer patterns of leaf synchrony and herbivory within related species, assuming that they shared herbivores. We simultaneously monitored leaf production and herbivory rates of five pairs of tree species, each composed of a specialist of terra firme or white-sand forests within the same lineage. Our prediction was not supported by the strong interaction of habitat and lineage for leaf synchrony within individuals of the same species; although habitat specialists differed in leaf synchrony within four of five lineages, the direction of the effect was variable. All species showed short time lags for the correlation between leaf production and herbivory, suggesting that insects are tightly tracking leaf production, especially for the most synchronous species. Leaf synchrony may provide an important escape defense against herbivores, and its expression appears to be constrained by both evolutionary history and environmental factors.     

Effects of habitat transitions on rainforest bird communities across an anthropogenic landscape mosaic

We compared bird community responses to the habitat transitions of rainforest‐to‐pasture conversion, consequent habitat fragmentation, and post‐agricultural regeneration, across a landscape mosaic of about 600 km² in the eastern Australian subtropics. Birds were surveyed in seven habitats: continuous mature rainforest; two size classes of mature rainforest fragment (4–21 ha and 1–3 ha); regrowth forest patches dominated by a non‐native tree (2–20 ha, 30–50 years old); two types of isolated mature trees in pasture; and treeless pasture, with six sites per habitat. We compared the avifauna among habitats and among sites, at the levels of species, functional guilds, and community‐wide. Community‐wide species richness and abundance of birds in pasture sites were about one‐fifth and one‐third, respectively, of their values in mature rainforest (irrespective of patch size). Many measured attributes changed progressively across a gradient of increased habitat simplification. Rainforest specialists became less common and less diverse with decreased habitat patch size and vegetation maturity. However, even rainforest fragments of 1–3 ha supported about half of these species. Forest generalist species were largely insensitive to patch size and successional stage. Few species reached their greatest abundance in either small rainforest fragments or regrowth. All pastures were dominated by bird species whose typical native habitats were grassland, wetland, and open eucalypt forest, while pasture trees modestly enhanced local bird communities. Overall, even small scattered patches of mature and regrowth forest contributed substantial bird diversity to local landscapes. Therefore, maximizing the aggregate rainforest area is a useful regional conservation strategy.     

Bird predation enhances tree seedling resistance to insect herbivores in contrasting forest habitats

According to the associational resistance hypothesis, neighbouring plants are expected to influence both the insect herbivore communities and their natural enemies. However, this has rarely been tested for the effects of canopy trees on herbivory of seedlings. One possible mechanism responsible for associational resistance is the indirect impact of natural enemies on insect herbivory, such as insectivorous birds. But it remains unclear to what extent such trophic cascades are influenced by the composition of plant associations (i.e. identity of ‘associated’ plants). Here, we compared the effect of bird exclusion on insect leaf damage for seedlings of three broadleaved tree species in three different forest habitats. Exclusion of insectivorous birds affected insect herbivory in a species-specific manner: leaf damage increased on Betula pendula seedlings whereas bird exclusion had no effect for two oaks (Quercus robur and Q. ilex). Forest habitat influenced both the extent of insect herbivory and the effect of bird exclusion. Broadleaved seedlings had lower overall leaf damage within pine plantations than within broadleaved stands, consistent with the resource concentration hypothesis. The indirect effect of bird exclusion on leaf damage was only significant in pine plantations, but not in exotic and native broadleaved woodlands. Our results support the enemies hypothesis, which predicts that the effects of insectivorous birds on insect herbivory on seedlings are greater beneath non-congeneric canopy trees. Although bird species richness and abundance were greater in broadleaved woodlands, birds were unable to regulate insect herbivory on seedlings in forests of more closely related tree species.     

Early Response of Soil Properties and Function to Riparian Rainforest Restoration

Reforestation of riparian zones is increasingly practiced in many regions for purposes of biodiversity conservation, bank stabilisation, and improvement in water quality. This is in spite of the actual benefits of reforestation for recovering underlying soil properties and function remaining poorly understood. Here we compare remnant riparian rainforest, pasture and reforestation plantings aged 2–20 years in an Australian subtropical catchment on ferrosols to determine the extent to which reforestation restores key soil properties. Of the nine soil attributes measured (total nitrogen, nitrate and ammonium concentrations, net nitrification and ammonification rates, organic carbon, bulk density, fine root biomass and water infiltration rates), only infiltration rates were significantly lower in pasture than remnant riparian rainforest. Within reforestation plantings, bulk density decreased up to 1.4-fold and infiltration rates increased up to 60-fold with time post-reforestation. Our results suggest that the main outcome of belowground processes of early reforestation is the recovery of the soils'' physical structure, with potential beneficial ecosystem services including reduced runoff, erosion and associated sediment and nutrient loads in waterways. We also demonstrate differential impacts of two commonly planted tree species on a subset of soil properties suggesting that preferential planting of select species could accelerate progress on specific restoration objectives.     

Biogeography of mutualistic fungi cultivated by leafcutter ants

Leafcutter ants propagate co‐evolving fungi for food. The nearly 50 species of leafcutter ants (Atta, Acromyrmex) range from Argentina to the United States, with the greatest species diversity in southern South America. We elucidate the biogeography of fungi cultivated by leafcutter ants using DNA sequence and microsatellite‐marker analyses of 474 cultivars collected across the leafcutter range. Fungal cultivars belong to two clades (Clade‐A and Clade‐B). The dominant and widespread Clade‐A cultivars form three genotype clusters, with their relative prevalence corresponding to southern South America, northern South America, Central and North America. Admixture between Clade‐A populations supports genetic exchange within a single species, Leucocoprinus gongylophorus. Some leafcutter species that cut grass as fungicultural substrate are specialized to cultivate Clade‐B fungi, whereas leafcutters preferring dicot plants appear specialized on Clade‐A fungi. Cultivar sharing between sympatric leafcutter species occurs frequently such that cultivars of Atta are not distinct from those of Acromyrmex. Leafcutters specialized on Clade‐B fungi occur only in South America. Diversity of Clade‐A fungi is greatest in South America, but minimal in Central and North America. Maximum cultivar diversity in South America is predicted by the Kusnezov–Fowler hypothesis that leafcutter ants originated in subtropical South America and only dicot‐specialized leafcutter ants migrated out of South America, but the cultivar diversity becomes also compatible with a recently proposed hypothesis of a Central American origin by postulating that leafcutter ants acquired novel cultivars many times from other nonleafcutter fungus‐growing ants during their migrations from Central America across South America. We evaluate these biogeographic hypotheses in the light of estimated dates for the origins of leafcutter ants and their cultivars.